Influenza is contagious. Viruses that cause influenza spread from person to person mainly by airborne droplets of respiratory fluids that are sent through the air when someone infected with the virus coughs or sneezes. Other people can inhale the airborne virus and become infected. In some cases, the flu can be spread when someone touches a surface (e.g., doorknobs, countertops, telephones) with the virus on it and then touches his or her nose, mouth, or eyes. The flu is most easily spread in crowded conditions such as schools.

There are 3 families of influenza virus: A, B, and C. Type C more commonly affects ducks, geese, turkeys, and chickens, but can be involved in a small percentage of human cases. Type B mainly affects humans, causes milder disease, and changes very little from year to year.

Type A influenza poses the most serious problems for humans. Strains of this type have been found in birds, humans, horses, pigs, seals, whales, and ferrets. Viruses that affect 2 different species sometimes combine, mixing and matching genetic information, to create a new strain against which nobody is immune and for which no vaccine has been prepared. There are an infinite number of possible new varieties of type A influenza.

Flu takes 1 to 4 days to incubate in humans, but infected people become contagious before symptoms appear, often just 24 hours after the virus enters the body. Adults remain infectious (can spread the virus to others) for about 6 days, and children remain infectious for up to 10 days.